AVR32: Get rid of board_early_init

board_early_init() is left over from some early prototyping work
where we had to initialize the SDRAM controller ourselves. This
depends on the kernel being loaded into static RAM, which just
isn't possible on any commercially available products today.

In order to run without a boot loader, we need to create a zImage
stub or have the debugger initialize the SDRAM for us (for really
low-level debugging)

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>